1. Field of the Invention
The present invention relates to a system for delivering video data through a network.
2. Description of the Prior Art
Improvements in hardware and software techniques have provided a way to handle data such as video data, which are difficult to handle as digital data, in the same manner as in the case of numerical data. Furthermore, as Internet and Intranet have become widespread, demands for delivery of such video data through a network have grown.
Most of networks which have been widely available as local area networks are in the form of common type networks such as Ethernet. These networks use CSMA/CD (Carrier Sense Multiple Access with Collision Detection) system mainly, and hence show the characteristics that increases the transmission delay time exponentially as the amount of data transferred increases.
When video data are delivered in real time, it is necessary to maintain a constant transfer bit rate to ensure the continuity in delivery of video data. Even when using a common type network which causes a transmission delay dependent upon the amount of data, the transfer bit rate can be maintained by using a communications protocol designed specially for video data to surely ensure continuity in the delivery of video data.
However, when delivering video data using a common type network, a mixture of the video data and other available data may cause difficulty in keeping the transfer bit rate of video data because of the traffic of the other available data even though a mechanism for maintaining the transfer bit rate of video data is added to the system. In such a case, continuity in the delivery of video data can not be ensured surely, and interrupts and disturbances are caused during replay of video data transferred.
Therefore, it is necessary to provide a cable intended for delivery of video data to prevent the video data from being mixed with other available data, in addition to provision of a mechanism for maintaining the transfer bit rate of video data, to surely ensure continuity in the delivery of video data when delivering the video data in real time using a common type network.
A method of reducing the amount of video data to be delivered when the bandwidth of a network required for video data transmission cannot be ensured sufficiently is disclosed in Japanese Patent Application Laid Open (KOKAI) No. 7-75092. In accordance with this method, when compressing video data to be delivered through a network, the amount of data is reduced by changing a spatial frequency region used according to a load imposed on the network. In addition, to ensure a constant bandwidth, a local area network using TDM (Time Division Multiplexer) is utilized as the network.
Referring now to FIG. 9, it illustrates a block diagram showing a prior art system for delivering video data which is applied to a network with CSMA/CD system, as disclosed in Japanese Patent Application Laid Open (KOKAI) No. 7-75092. In FIG. 9, reference numeral 201 denotes a terminal device, 204 denotes a network, 20 denotes a camera for inputting images, 22 denotes a pre-processing unit for, for example, digitizing an input signal, 23 denotes a discrete-cosine transformation unit, and 24 denotes a control unit for dynamically changing a spatial frequency. Furthermore, reference numeral 25 denotes a quantization unit, 26 denotes a variable-length coding unit, 21 denotes a microphone for generating an audio signal, 29 denotes a sound signal processing unit, 27 denotes an image/sound multiplexer, 28 denotes an interface unit between the terminal device and the network 204, 32 denotes a network monitoring unit, 31 denotes a control unit for controlling the upper limit of transmission frequency, and 30 denotes a unit for selecting transmission frequency components.
Next, a description will be made as to the operation. The pre-processing unit 22 and discrete cosine transforming unit 23 digitize and convert an image input through the camera 20 into components in the spatial frequency region. In accordance with a load imposed on the network 204 detected by the network monitoring unit 32, the transmission frequency upper limit controlling unit 31 and transmission frequency components selecting unit 30 judge and select frequency components to be transferred.
The quantization unit 25 quantizes the data about the range of frequency selected and the variable-length coding unit 26 encodes the quantized data into variable-length codes. The sound signal processing unit 29 digitizes an audio signal input through the microphone 21 and compresses corresponding digital data. The image/sound multiplexer 27 combines the data from the variable-length coding unit 26 and sound processing unit 29 to generate a set of video data using time division multiplexing technique. The interface unit 28 converts the video data signal into a form suitable for the network 204. Finally, the video data is delivered to the network 204.
Since the prior art video data delivering system having the structure mentioned above is based on the premise that video data input in real time and compressed are transmitted through a common type network such as Ethernet, there is a problem in that, when delivering video data compressed and stored at a constant bit rate, it is necessary to reproduce video data in a state previous to the compression and compress the video data again according to a load imposed on the network.